Role Of Convective Acceleration In The Interfacial Instability Of Liquid-Gas Coaxial Jets

Interfacial instabilities play a major role in breakup events in turbulent multiphase flow. Their role has been clearly identified for two-fluid atomization, and is of paramount importance in spray formation. In planar geometries, Kelvin-Helmholtz instabilities are the main mechanism of creation of a two-phase mixing layer, and information such as wavelengths and frequencies is available in the literature. In cylindrical geometries, the instabilities quickly become three-dimensional and thorough characterization is lacking, despite a wide range of applications using coaxial atomization. We conduct an experimental study of how the interfacial instabilities of a liquid jet surrounded by a turbulent gas co-flow accelerate and develop, before break-up and spray formation. We use high-speed shadowgraphy over a wide range of gas Reynolds numbers to compute the velocity of interfacial perturbations, using Lagrangian tracking, followed by a Eulerian conditioning to obtain local statistics. We identified two regimes of the gradient of the longitudinal mean velocity as a function of the gas Reynolds number: a quadratic scaling at low gas Reynolds numbers and a linear scaling at higher gas Reynolds number. In contrast, the transverse velocity gradients show a linear scaling with gas Reynolds number throughout the studied range.